L. Fernández Barquín

Resistivity changes of some amorphous alloys undergoing nanocrystallization

Abstract The electrical resistivity of amorphous alloys with compositions: Fe73.5Nb3Cu1Si13.5B9, Fe86Zr7Cu1B6 and Co80Nb8B12 has been studied in the temperature range from 300 to 1100 K, where crystallization occurs. The products of crystallization and the grain size have been studied by X-ray diffraction. In a first step, all the alloys crystallize with small grains of a few nanometers in diameter (nanocrystalline state), and the resistivity behavior at this process accounts for the difference between the amorphous and nanocrystalline phases. The nanocrystalline phases are: α-Fe-Si, α-Fe and fcc Co for the three compounds studied respectively. A second process, at which grain growth and precipitation of intermetallic compounds and borides takes place, has been found for all the alloys. The resistivity is sensitive, not only to the total transformed sample amount, but to the topological distribution of the crystalline phases, and therefore shows a more complex behavior than other well established techniques, as differential scanning calorimetry. This supplementary information given by the resistivity is also discussed.

Magnetic and transport properties of Fe-Zr-B-(Cu) amorphous alloys

FeZrB metallic glasses present magnetic properties that are enhanced compared to the pure FeZr ones. In particular, a large increase of the Curie temperature has been found. Magnetic and Mossbauer measurements show a decrease of the spin-glass character and a parallel homogenization of the hyperfine-field distribution as the boron concentration increases. Resistivity versus temperature measurements show a change in behaviour with B content: in the samples with small amounts of boron, a minimum in the resistivity versus temperature curves appears near the Curie temperature, while samples with high boron content show a low-temperature minimum, characteristic of most metallic glasses. The analysis of the results suggests that the evolution of the magnetic behaviour is related to changes in the density of states at the Fermi level, rather than to changes in the Fe - Fe distances. This is in agreement with published data on the specific heat of FeZr and FeB glasses. The influence of boron is shown to greatly enhance the weak itinerant ferromagnetism of FeZr glasses, leading to stronger ferromagnetic behaviour. The characteristic features of the resistivity are analysed in terms of localization effects on the conduction electrons, which extend to higher temperatures in the low-boron-content alloys.

Thermoelastic martensitic transformation in ferromagnetic Ni–Fe–Al alloys: Effect of site disorder

Thermoelastic martensitic transformation from a face-centered-cubic austenite phase to tetragonal martensite phase has been established in a new ternary ferromagnetic alloy system Ni–Fe–Al (“prepared” in different states of site disorder) based on the results of a detailed neutron diffraction, electrical resistivity, and magnetization studies. Ordered (annealed) Ni55Fe20Al25 has a high Curie temperature TC⪢300K and an extremely wide temperature range over which martensite and austenite phases coexist. By comparison, site-disordered (quenched) Ni55Fe20Al25 has a lower TC (=225K), higher ductility (shape memory effect), higher saturation magnetization, and a very well-defined martensitic transformation around TC.

Influence of boron on the magnetic and transport properties of FeZr amorphous and nanocrystalline alloys

The magnetic properties and electrical resistivity of amorphous and nanocrystalline FeZr and FeZrB(Cu) alloys are compared in a wide range of temperatures (4 to 1000 K). The addition of boron increases the Curie temperature of the alloys and induces a broad minimum in the resistivity vs temperatures. A first step of crystallization occurs around 700 K in all the alloys, giving rise to /spl alpha/-Fe crystallites of very small size. Small amounts of boron greatly influence the exchange interactions, enhancing the ferromagnetic character of these compounds. >

Crystal structure and magnetic behaviour of nanocrystalline Fe-Nb-Cu-Si-B alloys studied by means of in situ neutron diffraction

Two Fe-Nb-Cu-Si-B alloys, (B9) and (B6), prepared with the isotope, have been analysed using data obtained by means of in situ neutron diffraction. This technique allows one to scrutinize crystallographic phases during thermal treatments, avoiding problems due to sample handling. The B9 sample develops Fe(Si) nanometric crystals (10 nm) with 19 at.% Si in the phase when it is annealed at for one hour. An increase to favours the growth of Fe(Si) grains and the crystallization of other phases, mostly Fe borides. A Rietveld analysis of these phases results in a good reproduction of the nominal composition of the alloy. It also elucidates the crystallographic structure of the Fe(Si) phase. This is similar to the structure, but with some of the Fe atoms occupying some (45%) of the Si 4a sites. The compositions and amounts of the phases derived are in agreement with Mossbauer spectroscopy results for the same sample. Knowledge of the Fe(Si) composition enables one to compare the different magnetic behaviours observed for bulk and nanocrystalline alloys. By contrast, B6 alloy does not show the presence of a Fe(Si) structure, presumably due to the lower amount of Si in the Fe(Si) phase. The thermal expansion of the phases that appear is fairly linear and the corresponding thermal expansion coefficients for the different phases have been extracted. The magnetic structure of the Fe(Si) phase is ferromagnetic collinear, without any trace of antiferromagnetic ordering. The thermal variation of the (1, 1, 1) magnetic peak intensity of the Fe(Si) phase matches well with reported DC magnetization results.

Observation of isotropic-dipolar to isotropic-Heisenberg crossover in Co- and Ni-substituted manganites

High-precision ac susceptibility data have been taken on the La0.7Pb0.3Mn1−y(Co, Ni)yO3 (y=0, 0.1, 0.2 and 0.3) manganite system over a wide range of amplitudes and frequencies of the ac driving field in a temperature range that embraces the critical region near the ferromagnetic (FM)–paramagnetic (PM) phase transition (occurring at the Curie point TC). Elaborate data analysis was performed that (i) enabled the first observation of a crossover from a three-dimensional (3D; d=3) isotropic long-range dipolar asymptotic critical behavior to a d=3 isotropic short-range Heisenberg critical regime as the temperature is raised from TC in the compositions y≠0 (no such crossover is observed in the parent compound, y=0) and (ii) brought out clearly the importance of dipole–dipole interactions between the eg electron spins and/or between eg–t2g electron spins in establishing long-range FM order in the insulating state. The final charge and spin states of Co and Ni ions, substituting for the Mn3+ and/or Mn4+ ions, are arrived at by using a scenario of substitution that is consistent not only with the present results but also with the previously published structural, thermo-gravimetric, bulk magnetization, dc magnetic susceptibility and electrical resistivity data on the same system. The marked similarity seen between the magnetic behavior of the manganite system in question and the quenched random-exchange ferromagnets, within and outside the critical region, suggests that the percolation model forms an adequate description of the FM metal-to-PM insulator transition.

Interfacial magnetic coupling between Fe nanoparticles in Fe–Ag granular alloys

The role of the interface in mediating interparticle magnetic interactions has been analysed in Fe50Ag50 and Fe55Ag45 granular thin films deposited by the pulsed laser deposition technique (PLD). These samples are composed of crystalline bcc Fe (2–4 nm) nanoparticles and fcc Ag (10–12 nm) nanoparticles, separated by an amorphous Fe50Ag50 interface, occupying around 20% of the sample volume, as determined by x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), and high resolution transmission electron microscopy (HRTEM). Interfacial magnetic coupling between Fe nanoparticles is studied by dc magnetization and x-ray magnetic circular dichroism (XMCD) measurements at the Fe K and Ag L2,3 edges. This paper reveals that these thin films present two magnetic transitions, at low and high temperatures, which are strongly related to the magnetic state of the amorphous interface, which acts as a barrier for interparticle magnetic coupling.

Non-dipolar magnetic coupling in a strongly interacting superparamagnet: nanogranular Fe26Cu8Ag66

Unambiguous evidence for non-dipolar, as well as dipolar, inter-granular interactions has been found in the strongly interacting superparamagnetic mechanical alloy Fe26Cu8Ag66. This alloy comprised 5-6 nm BCC Fe-Cu grains in a matrix of similarly sized FCC Ag grains, and was of excellent purity and homogeneity. Magnetic relaxation was measured via DC magnetometry, AC susceptibility, muon spin relaxation and Mossbauer effect experiments. A phenomenological model in which magnetic interactions increased the apparent size of the grains, while retaining the Arrhenius law behaviour of non-interacting grains, predicted the observed variation in blocking temperature as a function of measurement time, with an apparent grain size d* approximately three times larger than the actual size. Significant differences were found in comparisons of temperature-dependent hysteresis data and the predictions of a dipolar interactions model, indicating the additional presence of non-dipolar interactions, most likely RKKY, in the alloy. Corroborating evidence was found in a low-temperature spin-glass-like peak in AC susceptibility data, indicating the possible influence of isolated Fe atoms or clusters within the Ag matrix in mediating inter-granular RKKY interactions

Scrutinizing the role of size reduction on the exchange bias and dynamic magnetic behavior in NiO nanoparticles.

NiO nanoparticles (NPs) with a nominal size range of 2-10 nm, synthesized via high-temperature pyrolysis of a nickel nitrate, have been extensively investigated using neutron diffraction and magnetic (ac and dc) measurements. The magnetic behavior of the NPs changes noticeably when their diameter decreases below 4 nm. For NPs larger than or equal to this size, Rietveld analysis of the room temperature neutron diffraction patterns reveals that there is a reduction in the expected magnetic moment per [Formula: see text] ion with respect to bulk NiO, which is linked to the existence of a magnetically disordered shell at the NP surface. The presence of two peaks in the temperature dependence of both the dc magnetization after zero-field-cooling and the real part of the ac magnetic susceptibility is explained in terms of a core (antiferromagnetic, AFM)/shell (spin glass, SG) morphology. The high-temperature peak ([Formula: see text] K) is associated with collective blocking of the uncompensated magnetic moments inside the AFM core. The low-temperature peak ([Formula: see text] K) is a signature of a SG-like freezing of the surface [Formula: see text] spins. In addition, an exchange bias (EB) effect emerges due to the core/shell magnetic coupling. The cooling field and temperature dependences of the EB effect and the coercive field are discussed in terms of the core size and the effective magnetic anisotropy of the NPs. However, NiO NPs of 2 nm in size no longer show AFM order and the [Formula: see text] magnetic moments freeze into a SG-like state below [Formula: see text] K, with no evidence of EB effect.

Influence of the preparation conditions on the magnetic properties and electrical resistivity of Fe73.5Nb3Cu1Si13.5B9 nanocrystalline alloys

Abstract We present the magnetic properties and the resistivity values of a series of Fe73.5Nb3Cu1Si13.5B9 alloys quenched from different melting temperatures and using different spin velocities. Some nanocrystals with a strong preferred orientation can be obtained directly from the melt at low velocities. Such partially crystalline samples show a lower room-temperature magnetostriction and resistivity than the amorphous ones. They also show a retardation in the nanocrystallization process, but the magnetic properties are poorer than those of the high-velocity samples, both in the as-quenched state and after complete nanocrystallization.